docs: def validate_wavetable(self):
          """Enforce monotonic, ascending wavelength array with no zero or
          negative values.
  
          Raises
          ------
          pysynphot.exceptions.DuplicateWavelength
              Wavelength array contains duplicate entries.
  
          pysynphot.exceptions.UnsortedWavelength
              Wavelength array is not monotonic ascending or descending.
  
          pysynphot.exceptions.ZeroWavelength
              Wavelength array has zero or negative value(s).
  
          """
          # First check for invalid values
          wave = self._wavetable
          if N.any(wave <= 0):
              wrong = N.where(wave <= 0)[0]
              raise exceptions.ZeroWavelength(
                  'Negative or Zero wavelength occurs in wavelength array',
                  rows=wrong)
  
          # Now check for monotonicity & enforce ascending
          sorted = N.sort(wave)
          if not N.alltrue(sorted == wave):
              if N.alltrue(sorted[::-1] == wave):
                  # monotonic descending is allowed
                  pass
              else:
                  wrong = N.where(sorted != wave)[0]
                  raise exceptions.UnsortedWavelength(
                      'Wavelength array is not monotonic', rows=wrong)
  
          # Check for duplicate values
          dw = sorted[1:] - sorted[:-1]
          if N.any(dw == 0):
              wrong = N.where(dw == 0)[0]
              raise exceptions.DuplicateWavelength(
                  "Wavelength array contains duplicate entries", rows=wrong)